Cultivated dicotyledonous crops such as soybean, Brassica, and cotton have substantial commercial value throughout the world. The development of scientific methods useful in improving the quantity and quality of soybean and other crops is, therefore, of significant commercial interest. Significant effort has been expended to improve the quality of cultivated dicotyledonous crop species by conventional plant breeding. Methods of conventional plant breeding have been limited, however, to the movement of genes and traits between plant varieties.
In addition to traditional breeding techniques, incorporation of disease resistance, increased or modified oil content, and other desirable traits can be envisioned using the modern tools of molecular biology including plant genetic engineering. Plant genetic engineering involves the transfer of a desired gene or genes into the inheritable germline of crop plants such that those genes can be bred into or among the elite varieties used in modern agriculture. Gene transfer techniques allow the development of new classes of crop varieties with improved disease resistance, herbicide tolerance, and increased nutritional value.
Agrobacterium has been widely used for the transformation of plants. Agrobacterium is a soil born phytopathogen that integrates a nucleic acid molecule (i.e., T-DNA) into the genome of a large number of dicotyledonous plants. Agrobacterium-mediated transformation involves incubation of cells or tissues with the bacterium, followed by regeneration of plants from the transformed cells via a callus stage. The advantage of the Agrobacterium-mediated gene transfer is that it offers the potential to regenerate transgenic cells at relatively high frequencies without a significant reduction in plant regeneration rates. Moreover, the process of DNA transfer to the plant genome is defined. That is, the DNA does not normally undergo any major rearrangements, and it integrates into the genome often in single or low copy numbers. Inoculation of a plant tissue with Agrobacterium is a disruptive process that can trigger a hypersensitive response in the tissue. As a result, the target tissue may become necrotic and the overall survival rate of transformants can be limited.
Accordingly, there remains a need for improved transformation methods to promote the engineering of desirable traits into agronomically important crops. In addition, there remains a need for highly efficient transformation methods that yield regenerable plant tissue.